1. Field of the Invention
The present invention is directed to the conversion of CO2-rich natural gases into liquid fuels. In particular, the invention is directed to reducing CO2 levels in CO2-rich natural gases that are converted into liquid fuels.
2. Description of the Related Art
The conversion of remote natural gas assets into transportation fuels has become more desirable because of the need to exploit existing natural gas assets as a way to satisfy the increasing need for transportation fuels. Generally, the term “remote natural gas” refers to a natural gas asset that cannot be economically shipped to a commercial market by pipeline.
Conventionally, two approaches exist for converting remote natural gases into conventional transportation fuels and lubricants including, but not limited to, gasoline, diesel fuel, jet fuel, lube base stocks, and the like. The first approach comprises converting natural gas into synthesis gas by partial oxidation, followed by a Fischer-Tropsch process, and further refining resulting Fischer-Tropsch products. The second approach comprises converting natural gas into synthesis gas by partial oxidation, followed by methanol synthesis wherein the synthesized methanol is subsequently converted into highly aromatic gasoline by a Methanol To Gasoline (MTG) process. Both of these approaches use synthesis gas as an intermediate. Also, while other approaches exist for using natural gas in remote locations, such approaches do not produce conventional transportation fuels and lubricants, but instead produce other petroleum products including, but not limited to, liquified natural gas (LNG) and converted methanol.
The Fischer-Tropsch and MTG processes both have advantages and disadvantages. For instance, the Fischer-Tropsch process has the advantage of forming products that are highly paraffinic. Highly paraffinic products are desirable because they exhibit excellent combustion and lubricating properties. Unfortunately, a disadvantage of the Fischer-Tropsch process is that the Fischer-Tropsch process emits relatively large amounts of CO2 during the conversion of natural gas assets into saleable products. An advantage of the MTG process is that the MTG process produces highly aromatic gasoline and LPG fractions (e.g., propane and butane). However, while highly aromatic gasoline produced by the MTG process is generally suitable for use in conventional gasoline engines, highly aromatic MTG gasoline may be prone to form durene and other polymethyl aromatics having high crystallization temperatures that form solids upon standing. In addition, the MTG process is more expensive than the Fischer-Tropsch process and the products produced by the MTG process cannot be used for lubricants, diesel engine fuels or jet turbine fuels.
Catalysts and conditions for performing Fischer-Tropsch reactions are well known to those of skill in the art, and are described, for example, in EP 0 921 184A1, the contents of which are hereby incorporated by reference in their entirety. A schematic of a conventional Fischer-Tropsch process is shown in FIG. 1. In FIG. 1, a natural gas feed stream 11 comprising CH4 and CO2, enters a first separator 12 wherein an amount of CO2 is removed in an exit stream 13. A natural gas feed stream 14, comprising CH4 and CO2, exits the first separator 12 and mixes with a stream 15 of O2 and H2O. The feed stream 14 then enters a synthesis gas formation reactor 16. A synthesis gas stream 17, comprising CO, H2 and CO2, exits the synthesis gas formation reactor 16 and enters a Fischer-Tropsch reactor 18. A Fischer-Tropsch product stream 19 exits the Fischer-Tropsch reactor 18 and enters a second separator 20. The second separator 20 separates the Fischer-Tropsch product stream 19 into a hydrocarbon products stream 21, and an unreacted gas stream 22, comprising unreacted CO, H2 and CO2. The unreacted gas stream 22 either recirculates in a recirculation stream 24 that mixes with the synthesis gas stream 17 before the synthesis gas stream enters the Fischer-Tropsch reactor 18, or exits the process in an exit stream 23 where the unreacted gases are used as a fuel.
The Fischer-Tropsch process can be understood by examining the stoichiometry of the reaction that occurs during a Fischer-Tropsch process. For example, during Fischer-Tropsch processing, synthesis gas (i.e., a mixture including carbon monoxide and hydrogen), is generated, typically from at least one of three basic reactions. Typical Fischer-Tropsch reaction products include paraffins and olefins, generally represented by the formula nCH2. While this formula accurately defines mono-olefin products, it only approximately defines C5+ paraffin products. The value of n (i.e., the average carbon number of the product) is determined by reaction conditions including, but not limited to, temperature, pressure, space rate, catalyst type and synthesis gas composition. The desired net synthesis gas stoichiometry for a Fischer-Tropsch reaction is independent of the average carbon number (n) of the product and is about 2.0, as determined by the following reaction equation:nCO+2nH2nH2O+nCH2where nCH2 represents typical Fischer-Tropsch reaction products such as, for example, olefins and paraffins.
The three general reactions that produce synthesis gas from methane are as follows:    steam reforming of methane: CH4+H2O CO+3H2;    dry reforming, or reaction between CO2 and methane: CH4+CO2 2CO+2 H2; and    partial oxidation using oxygen: CH4+½O2 CO+2H2.
Although the above general reactions are the basic reactions used to produce synthesis gas, the ratio of hydrogen to carbon monoxide produced by the above reactions is not always adequate for the desired Fischer-Tropsch conversion ratio of 2.0. (In the present application all ratios are molar ratios, unless otherwise noted.) For example, in the steam reforming reaction, the resulting ratio of hydrogen to carbon monoxide is 3.0, which is higher than the desired hydrogen to carbon monoxide ratio of 2.0 for a Fischer-Tropsch conversion. Similarly, in the dry reforming reaction, the resulting hydrogen to carbon monoxide ratio is 1.0, which is lower than the desired hydrogen to carbon monoxide ratio of 2.0 for a Fischer-Tropsch conversion. In addition to exhibiting a hydrogen to carbon monoxide ratio that is lower than the desired ratio for a Fischer-Tropsch conversion, the above dry reforming reaction also suffers from problems associated with rapid carbon deposition. Finally, because the above partial oxidation reaction provides a hydrogen to carbon monoxide ratio of 2.0, the partial oxidation reaction is the preferred reaction for Fischer-Tropsch conversions.
In commercial practice, an amount of steam added to a partial oxidation reformer can control carbon formation. Likewise, certain amounts of CO2 can be tolerated in the feed. Thus, even though partial oxidation is the preferred reaction for Fischer-Tropsch conversions, all of the above reactions can occur, to some extent, in an oxidation reformer.
It is also important to provide a low sulfur gas feedstock for the partial oxidation reformer. Typically, this can be done by use of an adsorption or absorption process or combination thereof. Suitable adsorbents can include, for example, water, amines, caustic compounds, combinations thereof and the like. Suitable adsorbents can include, for example, ZnO, Cu, Ni, combinations thereof and the like. ZnO is a preferred adsorbent because it selectively removes sulfur species without removing CO2.
During partial oxidation, CO2 forms because the reaction is not perfectly selective. That is, some amount of methane in the reaction will react with oxygen to form CO2 by complete combustion. The reaction of methane with oxygen to form CO2 is generally represented by the following reactions:CH4+O2CO2+2H2andCH4+2O2CO2+2H2O.
Furthermore, steam added to the reformer to control coking, or steam produced during the Fischer-Tropsch reaction can react with CO to form CO2 in a water gas shift reaction represented by the following general reaction:CO+H2OCO2+H2.
Thus, invariably a significant amount of CO2 is formed during the conversion of methane into transportation fuels and lubricants by the Fischer-Tropsch process. The CO2 produced during the Fischer-Tropsch process exits the Fischer-Tropsch process in a tail gas exiting the Fischer-Tropsch unit. Tail gases exiting a Fischer-Tropsch process comprise any gases that remain unconsumed by the Fischer-Tropsch process.
The above equations represent general stoichiometric equations; they do not reflect an optimum synthesis gas composition for the kinetics or selectivity of a Fischer-Tropsch reaction. Moreover, depending on the nature of the Fischer-Tropsch catalyst, synthesis gas ratios other than 2.0, typically less than 2.0, are used to prepare the feed to a Fischer-Tropsch unit. However, because Fischer-Tropsch units typically produce products exhibiting a hydrogen to carbon monoxide ratio of about 2.0, the limiting reagent, typically H2, is consumed first. The extra reagent, typically CO, is then recycled back to the Fischer-Tropsch unit for further conversion. Synthesis gas compositions having hydrogen to carbon monoxide ratios other than 2.0 are typically generated by recycling unused reagents.
Because CO2 in the natural gas feedstock has a tendency to react in a dry reforming reaction, which suffers from the disadvantages of generating low hydrogen content synthesis gas and carbon deposits, only a limited amount of CO2 can be tolerated in the synthesis gas. Typically, the amount of CO2 in the synthesis gas must be limited to a few percent, preferably about 5 mol % or less. In instances where the synthesis gas comprises greater than 5 mol %, excess CO2 must be removed from the synthesis gas and destroyed. Suitable CO2 disposal methods include, but are not limited to, venting, injection into an underground reservoir, conversion to solid carbonates or injection into a body of water. Unfortunately, each of these methods suffer from disadvantages. First, venting is undesirable because it increases a facility's greenhouse gas emissions. Also, while injection into an underground reservoir and conversion into solid carbonates avoid additional greenhouse gas emissions, these methods are extremely costly. Finally, the injection of CO2 into a body of water, using conventional methods wherein CO2 is recovered at near atmospheric pressure, is both expensive and unproven.
As a result, there is an urgent need for a process that can convert CO2-rich natural gases into liquid fuels while economically reducing the amount of CO2 in the CO2-rich natural gases.